1. Field of the Invention
This invention relates to a method of making a rechargeable manganese-oxide based compound and a related electrode material. The compound disclosed may be combined with suitable additional materials and components to form a positive electrode for a rechargeable battery cell. The compound of the present invention can also be used with commercially available electrode materials such as, for example, electrolytic manganese dioxide and when it is so used, the product of the present invention imparts rechargeability to the entire material.
2. Background of the Invention and Description of the Prior Art
Manganese dioxide has been used extensively in primary battery cells and particularly in dry and alkaline manganese dioxide-zinc cells. These battery cells are generally made in a charged state, are used once and are then discarded. Recently, in view of both expanding energy requirements and environmental concerns about proper waste disposal of used battery cells, great emphasis has been placed upon the development of readily rechargeable battery cells which can be used in a wide range of equipment and consumer products. Rechargeable batteries would be useful in many applications such as the electric vehicle and consumer electronic products. Examples of other areas particularly suited to the use of rechargeable batteries include satellites, space travel equipment and as backups for solar energy systems.
As noted hereinbefore, manganese dioxide has been widely used in primary battery cells. It is desirable to continue to use manganese dioxide in batteries because of its low cost and its stability. Further, unlike other rechargeable batteries, manganese-oxide/zinc batteries do not generally contain toxic components. The commonly used form of manganese dioxide itself is not readily rechargeable. Furthermore, rechargeability is essentially impossible if the material has been deep discharged to greater than 50 percent of the theoretical two-electron capacity.
Although it has been shown that in certain circumstances electrolytic manganese dioxide may be recharged in the range of 100 times, this is only true if the material is discharged to less than 30 percent of the theoretical one-electron capacity (which, as would be understood by one skilled in the art, is equivalent to less than 15 percent of the theoretical two-electron capacity.) Kordesch, et al. Electrochemica. Acta, 26, 1495 (1981). This is a restriction which requires shallow discharge and which severely limits the amount of energy which can be retrieved from the cell. It also reduces the rechargeability or eliminates rechargeability entirely if the manganese dioxide is accidentally discharged beyond the limit. Thus, there is very little practical application to the rechargeability.
Various attempts have been made at making a rechargeable compound containing manganese dioxide. For example, U.S. Pat. No. 4,250,005 discloses a chemical process to prepare a compound which includes manganese dioxide doped with bismuth and/or lead. The resulting compound is rechargeable, however, it has a lower density than is desirable in many commercial applications. Density can be a critical factor in battery electrode materials because of space limitations in battery cells. In a given space, a more dense material produces more energy than would a material of lower density such as the material disclosed in this prior art patent. Furthermore, the method disclosed in U.S. Pat. No. 4,250,005 is a batch method involving several steps and a substantial amount of time. The steps include first mixing manganese nitrate and bismuth nitrate in a solution and thereafter maintaining a low temperature by placing the solution in an ice bath. Sodium hydroxide is then added with continuous stirring while maintaining the constant low temperature of about 2 degrees Celsius. The resulting suspension is oxidized by bubbling oxygen through the mixture for about 22 hours. This method involves the requirement that the reactive materials be kept at a low temperature on the order of about 1 to 2 degrees Celsius during the reaction. Such low temperature requires continuous cooling which is typically not feasible for large scale commercial applications. Further, and in addition to its low density, the material produced may not be commercially feasible because of its high resistivity.
In another prior an approach, manganese dioxide is physically mixed with oxides of bismuth, lead or mixtures thereof. Wroblowa, et al., Journal of Electroanalytical Chemistry, 238, 93 (1987). An electrode made in accordance with this method and which included manganese dioxide doped with bismuth oxide was shown to be rechargeable up to 250 cycles, but with a continuous drop in capacity as the number of cycles increased. In addition, this physical mixing process is time consuming and may lead to non-homogeneous mixtures, thereby affecting the performance of the electrodes. Moreover, the electrodes prepared with physical mixing require several electrochemical cycles for activation and use. The resulting material is not suitable for sealed cells because of the problem of gas evolution in complete cells, as discussed hereinafter with respect to other prior art methods.
Another method is disclosed in U.S. Pat. No. 5,156,934, which is owned by the assignee of the present invention. In that patent, commercially available electrolytic manganese dioxide or commercially available chemical manganese dioxide is used. The particles of the prepared electrolytic or chemical manganese dioxide are coated with bismuth hydroxide or in another embodiment of the method, the particles are coated with lead hydroxide. The electrode prepared with the bismuth-coated product shows good rechargeability and sufficient density and conductivity for commercial application. However, the material prepared in accordance with this method does not appear to be useful in sealed battery cells. Typically, a zinc anode and commercially available separator materials would be used. When using separator materials, which are recommended for use in alkaline batteries with potassium hydroxide or a mixture of potassium hydroxide and potassium fluoride as the electrolyte, gassing has been found to occur. This gassing is due to a reaction which occurs involving the unreacted bismuth oxide or bismuth hydroxide in the cell. More specifically, a finite amount of the bismuth oxide or hydroxide is soluble in the alkaline electrolyte. The soluble bismuth species pass through the commercial separator to the anode compartment where it could deposit on the zinc electrode and then produce a gas. The gas can build up within the cell which presents an undesirable situation. Thus, the material prepared by the method disclosed is not ideal for use in sealed cells.
U.S. Pat. No. 5,156,934 also describes additional references which pertain generally to the use of manganese dioxide in battery cells. U.S. Pat. No. 5,156,934 is presently herein incorporated by reference in its entirety.
U.S. patent application Ser. No. 07/645,984, filed Jan. 21, 1991 (now allowed) which is owned by the assignee of the present invention, discloses a method of preparing a rechargeable modified manganese-containing electrode material by electrolytic deposition. This method involves simultaneous deposition of a charged product on one electrode and of a discharged product on the counter electrode used during electrolysis. The products can then be used as electrode materials in rechargeable cells. In contrast, the method of the present invention involves fewer variables to be controlled during preparation and a shorter preparation time which are relevant factors in many applications. Furthermore, as discussed fully herein, the material prepared in accordance with the present invention can be mixed with commercially available materials to create a rechargeable product. In such a case, only a fraction of the material must be produced as compared to the amount of the prior art material which would need to be produced to manufacture a given rechargeable product.
None of the aforementioned prior art methods and procedures have disclosed a battery electrode material which is highly rechargeable and which has suitable properties of density and conductivity while still being useable in a sealed battery cell and which can be used to prepare relatively large quantities of the material in a short time and at low cost. Although some prior art materials exhibit good rechargeability, the previously disclosed materials generally do not lend themselves to practical commercial application. There remains a need for a battery electrode material which meets the aforementioned criteria for commercial application. There also remains a need for a material which can be used with commercially available materials such as, for example, electrolytic manganese dioxide and chemical manganese dioxide, and which when added to such commercially available products, will impart rechargeability to the material as a whole. There also remains a need for a method of making a suitable compound which is feasible for large scale commercial production of rechargeable battery cells.